The present invention pertains to a knee support or replacement appliance, comprising
at least one joint each consisting of at least one proximal section and at least one reciprocally articulated distal section,
fixtures for attaching at least one proximal section of the said joint to a femoral part of a lower limb and at least one distal section of the said joint to a tibial part of the said lower limb, and
guiding devices for the reciprocally articulated elements, allowing relative movement with flexion-extension, antero-posterior rolling in conjugation with slippage, rotation and/or varus between the said femoral and tibial parts, and thus allowing, looking laterally at the knee from an antero-posterior vertical plane, close-to-life reproduction of the physiological movement of the knee as achieved in the joint by the cruciate ligaments.
In terms of the invention, the term knee support appliance should be interpreted as a knee orthesis and the term replacement appliance as a knee prosthesis or endoprosthesis.
In the following document, the femoral and tibial parts of a lower limb refer not only to the parts of an existing limb but also the superior and/or inferior rigid sections replacing this limb.
The proximal section may therefore be adapted to a superior rigid section attached to the femur, or it may be extended for the length of the femur and be attached thereto, and the distal section may be adapted to an inferior rigid section attached to the tibia, or it may be extended for the length of the tibia and be attached thereto.
In the following document, flexion and extension refer to a relative movement between the femoral and tibial parts of a limb about a roughly horizontal axis traversing the knee schematically from left to right. Rotation refers to a relative movement between these parts about a vertical axis. Varus movement refers to a relative movement between these parts around an antero-posterior horizontal axis. Finally, rolling and antero-posterior slippage refer to a relative movement between the extremities of the tibial and femoral parts in an antero-posterior plane.
Ortheses are orthopaedic appliances which maintain movement and joint function of a deficient limb in a disabled person. Such appliances are made of two rigid parts, interlinked by a joint at the knee level, and fitted to one side or to both sides of the leg. The entire appliance may be attached with belts or moulds which run the length of the thigh and leg. Ortheses used for the lower limbs are customarily two symmetrical joints fitted at the knee, which allow them to move solely in flexion-extension with at best simultaneous antero-posterior displacement of the proximal part on the distal part (see EP-A-0297766, U.S. Pat. No. 3,779,654). Physiological movement of the knee is, however, three-dimensional, and these ortheses inevitably inevitably stress the ligaments, which defeats the purpose of any medical or surgical treatment.
We know from other sources that there are multiaxial ortheses fitted with grooved planar plates which operate in tandem, thus allowing the tibia to slide posteriorly in relation to the femur at the beginning of flexion followed by flexion without slippage (WO-A-92/15264).
There is also a polyaxial knee brace which is three-dimensional (see Orthotics Inc. Generation II polyaxial knee brace, Vancouver, Canada) and which comprises a joint which can be adjusted on one side of the deficient knee. In this joint, two spherical segments operate in tandem: they have two circularly arcuate grooves lying in an offset manner about an axis. In the initial degrees of flexion, the proximal section shifts, and once it reaches the limit of its grooved track, flexion continues over the groove on the second spherical segment, thus providing a three-dimensional if non-physiological movement. In fact, physiological movement of the knee, by means of the ligament rod system, comprises an antero-posterior rolling and slippage movement conjugated with rotation and varus movement. According to most medical studies, rotation and varus movement take place principally at the beginning of flexion, which this known applicance is not able to reproduce.
Another orthesis is known in which the guiding device is composed of grooved spherical shells which operate one inside the other (U.S. Pat. No. 5,107,824). In view of the fact that the center of the shells of the external joint of this appliance is situated at another position from the center of the shells of the internal joint the appliance is inevitably bound to block since the external and internal shells are also integrally interlinked by means of rigid belts around the thigh and leg. Moreover, the spherical shape of the shells and the fact that one is very snugly apposed to the other prevents a complex trajectory movement being obtained which mimics physiological movement of the knee.
Protheses are orthopaedic appliances which allow persons who have partially or totally lost their leg to remain ambulant. Fitted with a socket at the top and a joint at the level of the axis of the knee enabling flexion of the prosthesis, these appliances allow the disabled person to put weight on his stump. For femoral protheses for long amputation or amputation through the knee joint, it is impossible to position the prosthetic joint at its physiological site. The prosthetic joint is placed lower than the physiological axis of the knee, which makes walking uncomfortable and makes sitting look unaesthetic, since the thigh section is abnormally longer than the tibial section.
Finally, knee support or replacement appliances as described at the beginning of this document (see WO 94/09729) are known. These appliances, by means of a special configuration of the weight-bearing surfaces of the guiding devices of each joint, enable mutual movement of the articulated elements in the same way as is produced by a crossed rod system. Applied to curved, complex surfaces, it allows flexion to incorporate movements of rotation and varus simultaneously which vary in amplitude in proportion to the degree of flexion-extension.
During experiments with previously known appliances it emerged that, even in the best of the previously proposed solutions, joints were subject either to blocking of the guiding devices or to uncontrolled slippage between the articulated elements. This gives rise to excessively high wear of the guiding surfaces and to forcing of the ligaments in order to overcome the blocking of the mechanisms.
It is important to note that, in the present invention, a distinction is made between two types of xe2x80x9cslippagexe2x80x9d : xe2x80x9cantero-posterior slippagexe2x80x9d which has already been defined above and which involves a relative movement between tibia and femur, and xe2x80x9cuncontrolled slippage of articulated elementsxe2x80x9d which relates solely to parts of the appliance.
The object of the present invention is to develop a knee support or replacement appliance such as would overcome the previously mentioned drawbacks and in particular aid the guiding of articulated elements in order to make it impossible for articulated elements to block or for reciprocal uncontrolled slippage to occur, while allowing physiological and relative movement between the femoral and tibial parts, in particular their antero-posterior slippage.
To solve these problems we have provided, according to the invention, a knee support or replacement appliance as described at the outset of the document, this appliance comprising in addition
on at least one proximal section of at least one joint, a first engagement surface which extends over the length of a trajectory corresponding to a displacement from an instantaneous centre of movement formed by a projection of a crossover of the said ligaments in the said at least one proximal section during the said relative movement, and
on at least one distal section of the said at least one joint, a second engagement surface which extends over the length of a trajectory corresponding to the displacement from the said instantaneous centre of movement formed by a projection of the said crossover of the said ligaments in the said at least one distal section during the said relative movement, and which is in apposition to the first engagement surface during this relative movement,
and retention devices which prevent mutual slippage between the first and second engagement surfaces during the relative movement.
As has already been outlined, physiological movement of the knee involves several simultaneous movements, which is made possible by a special configuration of the bony extremities and by the fact that they are held together by the cruciate ligaments.
The appended FIG. 1 shows the antero-posterior plane of a knee G, seen from the side. In the position shown, the crossed rods AO and BC schematically represent the abovementioned cruciate ligaments with the leg in the extended position. The letter a indicates the posterior cruciate ligament and the letter c the anterior cruciate ligament. The letter b (AB) represents the intercondylar roof, and the letter d (CO) the tibial plateau. The right part of the diagram is behind knee G, and is shown with a broken line.
The intercondylar roof AB is part of what is called in orthopaedic jargon the xe2x80x9cfixed planexe2x80x9d and the tibial plateau OC is part of what is called the xe2x80x9cmobile planexe2x80x9d.
FIG. 1 shows three different displacement positions of the fixed plane relative to the mobile plane during relative movement of the knee, the mobile plane remaining stationary in this illustration.
As can be seen, the instantaneous centre of rotation of the fixed and mobile planes is the intersection I of the rods AO and BC. During displacement of the fixed plane as illustrated in FIG. 1 a curve is described by the successive positions of I in the mobile plane. This curve is termed the base and is marked IB.
This figure also shows the displacement positions of the mobile plane in relation to the fixed plane during relative movement of the knee, the fixed plane remaining stationary. Intersection I of the rods AO and BC thus takes up successive positions along a curve, termed the rolling curve, which is marked IR.
During mutual displacement of both planes, the rolling curve therefore rolls on the base without slippage over the kinematic plane: this results solely from the intersection of the crossed rods of the rod system which holds these planes together.
If these theoretical observations are applied to a knee joint, they signify that only the intersection of the cruciate ligaments produces a pure rolling movement. The further the centre of movement from this intersection the greater the amount of slippage in relation to rolling.
We have therefore planned according to the invention to apply to the joint(s) of the support or replacement appliance used proximal and distal sections, one moving on the other by means of engagement surfaces to the trajectories corresponding to the base and to the rolling curve of the movement of the cruciate ligaments. Moreover, to hinder any slippage between these surfaces, retention devices are provided. These devices and the trajectory of the engagement surfaces thus compel the instantaneous centre of the virtual rod system of their joint, which is a projection from the instantaneous centre of the cruciate ligaments in the sections of this joint, to follow a pure rolling trajectory: this avoids any inadvertent mutual slippage of the articulated elements and allows control of femoral and tibial slippage movements necessitated by the antero-posterior rolling of the relative movement of the knee.
According to one embodiment of the invention, the retention devices comprise the first enmeshing means mounted on the said first engagement surface on at least one part of this surface and the second enmeshing means mounted on the said second engagement surface on at least one part of this surface, and operating in tandem with the first enmeshing means during the said relative movement.
It should be noted that, in the case of enmeshing means operating in tandem, it may be considered, according to the invention, that the engagement surfaces of two proximal and distal sections are virtual and situated at the pitch lines of the enmeshing means.
According to an advantageous embodiment of the invention, the said first and second enmeshing means comprise cogs operating together in which a base of the teeth and an interdental space increase transversally to the said trajectory, moving from the inside to the outside of the joint.
As explained in detail in WO 94/09729, it is preferable that the two joints on either side of an antero-posterior plane should be different in order to enhance rotation and varus of the knee. As both these movements occur essentially at the beginning of flexion, it is therefore preferable that the teeth, in any event when the cogs start to engage, have a shape widening out towards the exterior.
According to a particular embodiment of the invention, the said first and second engagement surfaces are smooth. The retention devices may then advantageously comprise at least one belt presenting a first end and a second end, the first end of the said at least one belt being attached to one end of the said at least one proximal section close to an extremity of the said first engagement surface, the second end of the said at least one belt being attached to one end of the said at least one distal section close to an extremity of the said second engagement surface, situated facing the extremity of the first engagement surface to which the said at least one belt is attached to a proximal section.
Although it is not a prerequisite that the articulated elements of the appliances according to the invention allow rotation and varus, it is clearly preferable that they do so. Provision may therefore be made, as described for example in WO 94/09729, for the said guiding devices of each joint to comprise at least one guiding surface of predetermined curvature in an articulated element and at least one tracker element on the other articulated element and for these articulated elements to also have lateral appositional surfaces of predetermined curvature which operate together during the said relative movement between the femoral and tibial parts. It is to advantage that the said lateral appositional surfaces have mutually different complex curvatures by which they come only into partial contact, at least three points, during the movement between the femoral and tibial parts.
According to an improved embodiment of the invention, the trajectories of the said first and second enmeshing means extend in such a way as to fit the curvature of the respective articulated element on which they are fitted. The enmeshing means tend not to form rectilinear tracks over their entire length; in fact they follow complex curves corresponding to those set by the joint guiding devices, and do so in such a way as not to hinder the fairly physiological movement conferred by the guiding devices, while hindering any inadvertent slippage between the articulated elements.
Other embodiments of the invention are displayed in the claims appertaining to claim 1.
Other details and special features of the invention will appear in the description provided below in a non-limiting manner and with reference to the appended drawings.